Generally the prior art reports that the transesterification of aliphatic hydroxy compounds with carbonic acid, aliphatic diesters and aromatic diesters occurs readily in the presence of a basic catalyst and is a convenient method of synthesis of higher carbonates.
Several references deal with the transesterification of glycol carbonates using an aliphatic alcohol. Most demonstrate the use of methanol and ethylene carbonate.
U.S. Pat. No. 4,307,032 discloses a process for the preparation of a dialkyl carbonate by contacting a glycol carbonate of a 1,2-diol containing 2 to 4 carbon atoms with a selected alcohol to form the corresponding carbonate of said alcohol at a temperature of between 50.degree. and 250.degree. C., in the presence of an improved catalyst which is a thallium compound, allowing the reaction to take place under milder conditions. Thallium is however expensive and very toxic.
In another process disclosed in U.S. Pat. No. 4,181,676 there is taught a method for preparation of dialkyl carbonate by contacting a glycol carbonate of a 1,2-diol having 2 to 4 carbon atoms with a selected group of alcohols at an elevated temperature in the presence of an alkali metal or alkali metal compound wherein the improvement comprises employing less than 0.01 percent by weight of alkali metal or alkali metal compound based on the weight of the reaction mixture.
It is known that alkyl carbonates of the type ROCOOR can be obtained from alcohols and cyclic carbonates corresponding to the above formula through a transesterification reaction in the presence of alkali alcoholates or hydrates; however, moderate amounts of inorganic compounds are produced by these reactions and must be removed by methods which may unfavorably affect the general economy of the process.
In U.S. Pat. No. 4,062,884 this efficiency problem was addressed and it was found that dialkyl carbonates can be prepared by reacting alcohols with cyclic carbonates in the presence of organic bases, which makes it unnecessary to remove inorganic compounds and allows the catalyst to be totally recovered by means of simple distillation. The preferred organic base is a tertiary aliphatic amine.
U.S. Pat. No. 4,349,486 teaches a monocarbonate transesterification process comprising contacting a beta-fluoroaliphatic carbonate and a compound selected from the class of monohydroxy aliphatic alcohols, monohydroxy phenols and ortho-positioned dihydroxy aromatic compounds in the presence of a base. This invention claims to greatly reduce undesirable side reactions and only small amounts of carbonic acid-aliphatic-aromatic mixed diester are associated with the isolated aromatic monocarbonate reaction.
The Gilpin and Emmons Patent, U.S. Pat. No. 3,803,201, discusses problems associated with the separation of the methanol, dimethyl carbonate azeotrope and teaches a solution wherein dimethyl carbonate is isolated from the azeotrope by a combination of low temperature crystallization and fractional distillation.
In another article in the J. Org. Chem. 49(b) 1122-1125 (1984) Cella and Bacon discuss the results of their work. Among other things, they found that the alkylation of alkali metal bicarbonate and carbonate salts with alkyl halides in dipolar aprotic solvents and phase-transfer catalysts produces alkyl carbonates in good yields. The major limitation of this method is the failure of activated aryl halides or electronegatively substituted alkyl halides to produce carbonates due to the facility with which the intermediate alkoxy carbonate salts decompose.
In Japanese Patent No. 4,028,542-B there is disclosed a method for preparation of dialkyl carbonates from cyclic carbonates by reacting an alcohol with a cyclic carbonate having the formula: ##STR1## where R and R' are H or alkyl, in the presence of an organic base catalyst such as a tertiary amine. The advantage is that organic products do not have to be removed and catalysts can be recovered by distillation.
Disadvantages of the methods discussed above include in many cases the fact that it is necessary to use a large amount of methanol feedstock relative to the amount of dimethyl carbonate produced. Side reactions to unwanted products and efficient separation of the desired products can also present problems. Also, in many cases, alkali metal halides are coproduced and these halides present disposal problems.
In applicant's related application, U.S. Ser. No. 06/815,954, now U.S. Pat. No. 4,691,041, there is disclosed a new route to dimethyl carbonate using a heterogeneous catalyst comprising specified classes of ion exchange resins. This improved method requires fewer moles of methanol per mole of dimethyl carbonate.
Related Ser. No. 06/891,093, now U.S. Pat. No. 4,661,609, produces good yields of dimethyl carbonate and ethylene glycol simultaneously in the presence of a homogeneous catalyst of zirconium, titanium or tin.
Applicant's Ser. No. 06/924,072, now abandoned, coproduces high yields of dimethyl carbonate and ethylene glycol under mild conditions, using perfluorinated ion exchange resins.
It would be a substantial advance in the art to devise an efficient process for co-producing dimethyl carbonate and ethylene glycol in improved yields using mild conditions. It would also be desirable if the catalyst system minimized side reactions and allowed for efficient separation of the product. In the instant process the concentrations of ethylene glycol (EG) and dimethyl carbonate (DMC) in the crude liquid products are as high as 13.5 wt % and 21.4 wt % respectively and the selectivity to dimethyl carbonate and ethylene glycol is up to greater than 98%. The concentrations of DMC and EG in the crude liquid product are close to equilibrium. The dimethyl carbonate produced by this novel process can be used as a gasoline extender.